In order to reduce transmission losses in coaxial cables, a low density dielectric is used as an intermediate insulator (having a minimum density equal to about 15% of the density of the dielectric in a coaxial cable having a solid dielectric). For example, solid polytetrafluoroethylene (PTFE) may be replaced by expanded PTFE, giving a density that is lower than that of solid PTFE. The relative permittivity of expanded PTFE is lower than that of solid PTFE. Consequently, to retain electrical characteristics that are identical to those of conventional cables, and in particular to retain a similar characteristic impedance (it being recalled that the characteristic impedance of a cable depends on the concentricity of the various parts of the cable, on the ratio of their diameters, and on their relative dielectric permittivities), it is necessary to reduce the ratio between the inside diameter of the outer conductor (i.e. generally the outside diameter of the intermediate dielectric) and the outside diameter of the inner conductor, which in practice means that the outside diameter of the inner conductor needs to be increased.
However, when a cable is in use, it is subjected to numerous bending stresses: there is an ever increasing demand for cables taking up as little room as possible for the purpose of saving space, particularly in aviation, military, space, etc. applications.
Thus, increasing the outside diameter of a solid and stiff metal internal conductor while simultaneously reducing the compression strength of a low density dielectric has the effect during bending of the central conducting core being locally decentered because of its stiffness. This gives rise to a harmful change in the characteristic impedance and thus in the electrical properties of the cable in question.
A cable of such a structure cannot be subjected to radii of curvature that are less than four to five times the outside diameter of the cable.
It might be thought that it would be possible to use a cable in which the stiff metal central conductor is replaced by a flexible rod made of dielectric material and covered in strips of metal. Such a structure is described in Patent FR-2 487 568.
However, the solution provided by such a structure cannot be transposed to very high frequencies (typically greater than 12 GHz) where very small diameter cables are used (outside diameter down to 6.5 mm), nor can it be transposed to high operating temperatures (about 125.degree. C. and higher). The cellular polyurethane used for forming the supporting rod described in the patent mentioned above cannot withstand temperatures higher than 80.degree. C.
In addition, using a strip of metal disposed lengthwise and possible welded to make the inner conductor gives rise to a structure which is stiff and which does not accept small radii of curvature: on bending, the inner conductor is damaged.
For cables operating at high frequencies (e.g. 200 MHz), the thickness of metal required for the inner conductor may be of the order of one hundredth of a millimeter (with the minimum thickness e being a function of frequency f in accordance with the following equation: ##EQU1## where .mu. is the permeability of the metal used and .sigma. is its conductivity). This cannot be obtained using the method of injecting polyurethane into a metal tube constituting the central core as described in the above-mentioned patent. It is not possible to make a metal tube having a wall thickness of a few hundredths of a millimeter that is capable of withstanding polyurethane injection. In practice, the cables described in the above-mentioned patent have diameters of more than about ten millimeters. Finally, it is not possible using conventional techniques to obtain a cable that accepts small radii of curvature while giving rise to low transmission losses, and being capable of operating at very high frequencies and at high temperatures.
The object of the present invention is thus to provide a low loss cable capable of accepting small radii of curvature and capable of being used at very high frequencies and under high temperatures.